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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 EST.

1	Using RCU to Protect Read-Mostly Arrays
2	
3	
4	Although RCU is more commonly used to protect linked lists, it can
5	also be used to protect arrays.  Three situations are as follows:
6	
7	1.  Hash Tables
8	
9	2.  Static Arrays
10	
11	3.  Resizeable Arrays
12	
13	Each of these three situations involves an RCU-protected pointer to an
14	array that is separately indexed.  It might be tempting to consider use
15	of RCU to instead protect the index into an array, however, this use
16	case is -not- supported.  The problem with RCU-protected indexes into
17	arrays is that compilers can play way too many optimization games with
18	integers, which means that the rules governing handling of these indexes
19	are far more trouble than they are worth.  If RCU-protected indexes into
20	arrays prove to be particularly valuable (which they have not thus far),
21	explicit cooperation from the compiler will be required to permit them
22	to be safely used.
23	
24	That aside, each of the three RCU-protected pointer situations are
25	described in the following sections.
26	
27	
28	Situation 1: Hash Tables
29	
30	Hash tables are often implemented as an array, where each array entry
31	has a linked-list hash chain.  Each hash chain can be protected by RCU
32	as described in the listRCU.txt document.  This approach also applies
33	to other array-of-list situations, such as radix trees.
34	
35	
36	Situation 2: Static Arrays
37	
38	Static arrays, where the data (rather than a pointer to the data) is
39	located in each array element, and where the array is never resized,
40	have not been used with RCU.  Rik van Riel recommends using seqlock in
41	this situation, which would also have minimal read-side overhead as long
42	as updates are rare.
43	
44	Quick Quiz:  Why is it so important that updates be rare when
45		     using seqlock?
46	
47	
48	Situation 3: Resizeable Arrays
49	
50	Use of RCU for resizeable arrays is demonstrated by the grow_ary()
51	function formerly used by the System V IPC code.  The array is used
52	to map from semaphore, message-queue, and shared-memory IDs to the data
53	structure that represents the corresponding IPC construct.  The grow_ary()
54	function does not acquire any locks; instead its caller must hold the
55	ids->sem semaphore.
56	
57	The grow_ary() function, shown below, does some limit checks, allocates a
58	new ipc_id_ary, copies the old to the new portion of the new, initializes
59	the remainder of the new, updates the ids->entries pointer to point to
60	the new array, and invokes ipc_rcu_putref() to free up the old array.
61	Note that rcu_assign_pointer() is used to update the ids->entries pointer,
62	which includes any memory barriers required on whatever architecture
63	you are running on.
64	
65		static int grow_ary(struct ipc_ids* ids, int newsize)
66		{
67			struct ipc_id_ary* new;
68			struct ipc_id_ary* old;
69			int i;
70			int size = ids->entries->size;
71	
72			if(newsize > IPCMNI)
73				newsize = IPCMNI;
74			if(newsize <= size)
75				return newsize;
76	
77			new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
78					    sizeof(struct ipc_id_ary));
79			if(new == NULL)
80				return size;
81			new->size = newsize;
82			memcpy(new->p, ids->entries->p,
83			       sizeof(struct kern_ipc_perm *)*size +
84			       sizeof(struct ipc_id_ary));
85			for(i=size;i<newsize;i++) {
86				new->p[i] = NULL;
87			}
88			old = ids->entries;
89	
90			/*
91			 * Use rcu_assign_pointer() to make sure the memcpyed
92			 * contents of the new array are visible before the new
93			 * array becomes visible.
94			 */
95			rcu_assign_pointer(ids->entries, new);
96	
97			ipc_rcu_putref(old);
98			return newsize;
99		}
100	
101	The ipc_rcu_putref() function decrements the array's reference count
102	and then, if the reference count has dropped to zero, uses call_rcu()
103	to free the array after a grace period has elapsed.
104	
105	The array is traversed by the ipc_lock() function.  This function
106	indexes into the array under the protection of rcu_read_lock(),
107	using rcu_dereference() to pick up the pointer to the array so
108	that it may later safely be dereferenced -- memory barriers are
109	required on the Alpha CPU.  Since the size of the array is stored
110	with the array itself, there can be no array-size mismatches, so
111	a simple check suffices.  The pointer to the structure corresponding
112	to the desired IPC object is placed in "out", with NULL indicating
113	a non-existent entry.  After acquiring "out->lock", the "out->deleted"
114	flag indicates whether the IPC object is in the process of being
115	deleted, and, if not, the pointer is returned.
116	
117		struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id)
118		{
119			struct kern_ipc_perm* out;
120			int lid = id % SEQ_MULTIPLIER;
121			struct ipc_id_ary* entries;
122	
123			rcu_read_lock();
124			entries = rcu_dereference(ids->entries);
125			if(lid >= entries->size) {
126				rcu_read_unlock();
127				return NULL;
128			}
129			out = entries->p[lid];
130			if(out == NULL) {
131				rcu_read_unlock();
132				return NULL;
133			}
134			spin_lock(&out->lock);
135	
136			/* ipc_rmid() may have already freed the ID while ipc_lock
137			 * was spinning: here verify that the structure is still valid
138			 */
139			if (out->deleted) {
140				spin_unlock(&out->lock);
141				rcu_read_unlock();
142				return NULL;
143			}
144			return out;
145		}
146	
147	
148	Answer to Quick Quiz:
149	
150		The reason that it is important that updates be rare when
151		using seqlock is that frequent updates can livelock readers.
152		One way to avoid this problem is to assign a seqlock for
153		each array entry rather than to the entire array.
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